Note: Descriptions are shown in the official language in which they were submitted.
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TWIN-FLOW BEATER MILL FOR PREPARING FIBROUS MATERIALS
Background of the Invention
The invention relates to a twin-flow beater mill
which enables fibrous materials to be prepared by disin-
tegration parallel to the fibers into intermediateproducts such as are required for further industrial
processing, for example, in the board or pulp industries.
Such fibrous materials occur as waste products, for
example in the wood-processing and wood-working
industries in the form of sawdust and planing shavings.
However, large amounts of fibrous materials are also
produced in the processing of annual plants, such as, for
example, in the sugar-cane industry with regard to the
so-called bagasse. Large amounts of fibrous waste
materials are also to be prepared using grinding
technology in the reuse of old paper.
Economic considerations dictate that the preparation
of these fibrous materials to produce intermediate
products capable of further processing be performed at
high rates of throughput and with a low specific enerqy
requirement. These operational conditions are fulfilled
in principle by the so-called twin-flow beater mill in
which the charging is performed in the axial center of a
cylindrical grinding surface, from where the flow of
material passes outwards through the annular grinding gap
formed between the active edges of the beater plates and
the grinding surface on two symmetrically axially opposed
helical surfaces, aided by the air flow caused by the
beater rotor. This mode of operation yields a high rate
of throughput in conjunction with optimum utilization of
the entire grinding surface area. In addition, it is
possible by the purposeful selection of design
parameters, such as grinding surface confiquration,
grinding gap width, number of beater plates and the like,
or else by the selection of suitable operational para-
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meters, such as speed of the beater rotor or influencingof the air flow, to conduct the grinding process in such
a way that the ground material is subjected only to as
much energy as is just sufficient for the targeted degree
of comminution of the respective material.
Such a twin-flow beater mill has been disclosed in
German Patent 1,909,022. It has a beater rotor, which is
fitted with beater plates and surrounded concentrically
by a cylindrical grinding surface, and whose rotor plates
which carrying the beater plates in the axial rotor
center form an axially charged guide duct shaped like an
annular disk which opens out peripherally onto the
grinding surface. Correspondingly cylindrical screen webs
which determine the targeted degree of fineness of the
ground material are arranged on both sides of the grind-
ing surface.
Although this type of mill has proved outstanding in
the comminution of numerous types of material,
substantial problems occur, especially with a damp
charge, in processing fibrous materials which contain
fractions that are overlong and, additionally, thin, that
is to say in the form of strands or strings. This
enables the use of this type of mill in the special
industrial field of the preparation of fibrous materials
only with additional complicated measures, if at all.
Thus, in the known twin-flow beater mill, the rear
edges of the beater plates, which bridge the guide duct,
which is shaped like an annular disk in its peripheral
opening-out region, act as a trap for the fibrous
fractions in the form of strands. As a result, it is
possible, especially with a damp charge, for them to
build up on the inside of the beater plates irregular
accumulations of material which cause eccentric
unbalanced masses, so-called unbalance, on the beater
rotor, the consequence of which is uneven running of the
machine. Moreover, in this way the beater rotor clogs up
gradually and over its circumference in an irregular
distribution, resulting in a pulsating flow of material
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in conjunction with a decreasing rate of throughput. The
known twin-flow beater mill has therefore had to be
frequently shutdown for the purpose of scraping the
beater rotor. However, even the installation of special
scrapers has not been able to provide a satisfactory
remedy here, despite a substantial outlay on design.
In addition, with the known twin-flow beater mill it
is not possible for the degree of disintegration of the
fibrous materials to be quickly adapted by simple
measures to changed operating conditions such as can be
caused in the event of a change in state of the charge or
as a consequence of changed requirements for further
processing. Because of the risk of blockage, it is not
possible to use the replaceable screen rings that are
arranged on both sides of the grinding surface and which
have proved themselves for determining the degree of
fineness in the case of granular ground material for
fibrous materials. As a result, it was necessary for the
retention time of the material on the grinding surface,
and thus its degree of disintegration, to be influenced
only by exchanging the grinding surface for one having a
different angular orientation of the grinding surface
ribs. Not only was this removal expensive and the
conversion time consuming, but it also required the
service personnel to have a high degree of practical
knowledge.
Summary of the Invention
It is therefore the object of the invention to render
the operating principle of the twin-flow beater mill
capable of use for the preparation of fibrous materials
in a manner protecting the fibers, and to be precise both
with regard to trouble-free feeding of material and with
respect to discharging material without difficulty in a
simple way capable of influencing the degree of
disintegration.
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The object is achieved by interruption of the beater
tools in the peripheral opening-out region of the annular
disk-shaped guide duct which creates an entirely free
passage of material to the grinding surface, and which
ensures that it is no longer possible for accumulations
of material to build up there. In addition, the retaining
rings bearing on both sides against the end faces of the
grinding surface form annular classifying chambers which
are free of internals and also guarantee a discharge of
material that is trouble-free and yet capable of being
influenced in a simple way with regard to the degree of
disintegration.
A further feature of the invention is the designed
configuration of the beater rotor so that it is possible
according to the invention for the beater tools to be
interrupted in structural terms in the opening-out region
of the guide duct shaped like an annular disk.
Since the width of the interruption of the beater
tools consisting of beater plates is only a fraction of
the width of the guide duct shaped like an annular disk,
the beater plates lose only a small part of their active
edges owing to the interruption, it being the case,
moreover, that the chamfers provided at the rear on the
beater plates ensure that no deposition of material can
form here.
The invention further includes a distribution disk
which projects into the disk-shaped guide duct shaped
like an annular disk, with the pneumatically charged
fractions receiving additional tangential motive impulses
by means of which they are hurled in a pinpointed fashion
into the region of the grinding surface.
This pinpointed relative centrifugal force can be
further increased by subdividing the distribution disk
into peripheral sectors that can optionally be bent out
of the plane of the disk and twisted into one another.
If the grinding surface projects on both sides beyond
the beater plates, the two classifying chambers formed by
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the retaining rings experience a corresponding axial
widening.
The setting of the retaining rings by means of
control elements that can be locked outside the machine
5housing also serves at the same time as an axial bearing
for the grinding surface, which can be hydraulically
pushed out of the housing in the axial direction for the
purpose of replacing the grinding surface. Because of
the additional retaining rings held in a position of
10readiness in the mill housing, the possibility exists of
influencing the classifying effect, and thus the degree
of disintegration, gradually during the operation.
Since, according to the invention, the retaining
rings arranged on both end faces of the grinding surface
15effect the classification, and thus the influencing of
the retention time of the material on the grinding
surface, the grinding surface can now be occupied by
axially directed, replaceable strips, ribs or the like.
This not only reduces the costs of producing the grinding
20surface, but also greatly simplifies the improvement work
on it.
Brief DescriPtion of the Drawings
An exemplary embodiment of the invention is
represented in the drawing, wherein:
25Figure 1 is an axial cross-sectional view of a twin-
flow beater mill configured according to the invention;
Figure 2 is an end view looking in the direction of
the arrow A of Fig. 1 and showing the configuration of
the beater rotor in more detail; and
30Figure 3 is an enlarged, fragmentary view showing the
grinding surface and adjacent elements in more detail.
Detailed DescriPtion of the Preferred Embodiments
On its front end face, the mill housing 1 has a door
2 that can swivel out and through which an inlet duct of
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socket 3 extends. The duct 3 merges inside the housing
1 into an expanding distributor cone 4, with the material
being preferably fed through the duct 3 and cone 4 in a
pneumatic fashion.
Supported in a floating mount in the housing 1 on the
rear housing wall is a beater rotor 5. It consists of a
rotor hub 6 which is connected to rotate with the drive
shaft 7 of a drive motor. Fixed to the rotor hub 6 is a
rotor hub plate 8, which is connected to an inner annular
disk or plate 9 and two outer annular disks or plates 10
by means of anchor bolts 11 and spacer bushings 12.
The hub plate 8 and the three annular disks 9 and 10
are fitted on their circumference with beater plates 13
whose outer edges cooperate with a stationary grinding
surface 14 which surrounds the beater rotor 5 concentri-
cally and defines therewith a grinding gap a (Fig. 3).
The hub plate 8 forms with the inner annular disk 9
a guide duct 15, shaped like an annular disk, which has
an axial width b and into whose central region the dis-
tributor cone 4 opens. In its peripheral region, the
guide duct 15 opens out onto the axial center of the
grinding surface 14, which is occupied by strips or ribs
16 that are distributed uniformly over its circumference
and extend in an axially parallel fashion.
The beater plates 13 are centrally interrupted to
form a width c in the peripheral opening-out region of
the guide duct 15 and provided at the rear edges thereof
adjacent the width c with corresponding chamfers 17.
Moreover, a distribution disk 18 which is fixed to the
rotor hub 6 is arranged in the central region of the
guide duct 15. As may be seen from Figure 2, the
distribution disk 18 is subdivided in its outer region
into sectors 19 which can optionally be bent out of the
plane of the screen about their chords 20 and can be
twisted into one another.
Bearing against the two end faces of the stationary
grinding surface 14 are retaining rings 21 and 26 which
have a retaining rim height h and therefore form with the
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two outer regions of the grinding surface 14 and the two
outer annular disks 10 of the beater rotor 5 two annular
classifying chambers 22 which have an axial extent d
(Fig. 3) and in which the beater plates 13, which project
laterally beyond the outer annular disks 10, function in
addition as classifier blades. The retaining ring 21 on
the drive side is provided with a plurality of control
elements 23, which are distributed on the circumference
and guided through the wall of the housing 1, and which
can be locked by means of jam nuts 24 on webs 25 provided
outside the housing.
A second retaining ring 21' with a higher
retaining rim h' is held in a position of readiness on
the drive side on the inner wall of the housing 1, and is
likewise locked by means of control elements 23' on the
webs 25. Consequently, as indicated by dot and dash lines
in Figure 3, the second ring 21' can additionally be set
on the first retaining ring 21 when a higher separation
efficiency of the classification is required.
On the inlet side, the retaining ring 26 engaging the
adjacent end face of the grinding surface 14 is held in
place by holding bolts 27 which are replaceably screwed
on the inside of the housing door 2. Since it is thus
possible to directly exchange a retaining ring when the
door 2 is opened, there is no need for an additional
retaining ring to be held in a position of readiness on
the inner wall thereof. However, this is also possible in
principle, if it is desired to influence the classifying
effect during operation.
Located on both sides of the two classifying chambers
22 are annular discharge chambers 2B which are spatially
connected to the common material outlet 29.
The twin-flow beater mill configured according to the
invention operates as follows. With the aid of the
ventilation effect caused by the beater rotor 5, the
fibrous material is pneumatically fed through the inlet
socket 3 to the beater mill, where it passes through the
widening distributer cone 4 into the guide duct 15 in the
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shape of an annular disk. Here, it impinges on the
distribution disk 18, from where it is hurled off
tangentially with an additional mechanical impulse in the
direction towards the axial center of the grinding
surface 14. As a consequence of the interruption c of
the beater plates 13 provided in the opening-out region
of the guide duct 15, and the chamfered rear edges 17,
the entire flow of material, that is to say the above-
mentioned blockage-prone fractions in the form of
strands, strings or threads, passes unhindered into the
central region of the grinding surface 14. Starting from
here, the flow of material then splits into two subflows
which move pneumatically in opposite axial directions. In
the process, the component of movement caused by the
beater plates 13 is superimposed on the axial component
of movement effected by the air flow, so that the two
subflows move on oppositely directed helical surfaces
through the annular grinding gap a bounded by the active
edges of the beater plates 13 and the cylindrical
grinding surface 14. In the process, they pass friction,
shear and turbulent zones of high intensity which arise
because the active edges of the beater plates 13 pass at
high speed by the strips or ribs 16 of the grinding
surface 14 at the grinding gap spacing a. The high-energy
shear and friction forces prevailing here effect, in
conjunction with the material particles passing the
grinding gap a in a tightly packed fashion, predominantly
autogenous comminution effects which comminute the
material chiefly parallel to the fibers, so that very
narrow, slim slivers or splinters are formed whose fiber
structure remains, however, largely undamaged.
After the grinding gap has been passed, the retaining
rings 21 and 26 bearing against the two end faces of the
grinding surface 14 deny the fibrous splinters produced
in this way free axial outlet. Rather, there is imparted
the two material flows arriving there paths of movement
that are helix spiral in shape that tend radially
inwards. As known from the theory of air classification,
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a state of equilibrium is set up on these spiral surfaces
between the centrifugal forces acting on the material
particles, on the one hand, and the drag forces, on the
other hand. As a consequence of the diminution of the
particle size in the case of comminution, the centrifugal
forces, depending on the particle volume, decrease in
accordance with the laws of nature approximately in
accordance with the third power of their magnitude, and
thus more rapidly than the drag forces, which depend on
the "shadow area" and decrease only approximately in
accordance with the second power. The consequence of this
is that as the degree of comminution progresses the drag
forces acting from the air flow on the particles
gradually exceed the centrifugal forces. Consequently,
the splinter-shaped particles are held in the region of
the grinding surface by these centrifugal forces acting
on them only until they are reduced to a size at which
the drag force exceeds the centrifugal force. Only then
are they entrained by the spiral flow prevailing in the
two classifying chambers 22 and discharged over the
retaining rim of the retaining rings 21 and 26 into the
two lateral discharge chambers 28, from where they then
pass into the common material outlet 29.
If, as a consequence of replacement of the charge or
in the event of changed requirements for further
processing, a smaller particle size or a higher separa-
tion efficiency is required, the second retaining ring
21' with higher retaining rim h', which is held on the
inside of the housing 1 in a position of readiness, can
be additionally set on the first retaining ring 21 by
means of its control element 23'.
